1. Technical Field
The present invention relates to a demodulation circuit and an optical disk drive, and more particularly to reproduction of a wobble signal.
2. Related Art
In an optical disk of record enable type, a recording track is wobbled, and a recording clock signal is generated from the frequency of wobbling. Address information about a disk is embedded in a wobble signal by means of variously modulating wobbles. Consequently, reproduction of the wobble signal is indispensable for effecting recording operation, and the quality of the wobble signal greatly affects recording quality.
In the meantime, as the capacity of a CD, a DVD, a Blu-ray, an HD-DVD, or an optical disk increases, a track pitch becomes smaller in proportion to the diameter of a beam spot focused on the disk. Crosstalk that a wobble signal of a track of interest receives from a wobble signal of an adjacent track tends to increase. Specifically, the phase of the wobble signal of a track being subjected to recording or reproduction is disturbed under the influence of crosstalk from a wobble signal of a track adjacent to an inner radius side or an outer radius side of the track being subjected to recording or reproduction. Wobble jitter increases as a result of variations in phase, which in turn induces an increase in an address demodulation error. For instance, in a Blu-ray, a wobble signal is multiplied by a clock signal that is generated by supplying the wobble signal to a PLL circuit, thereby performing MSK demodulation and HMW demodulation. However, a PLL clock signal cannot follow a disturbance in the phase of the wobble signal, and hence the crosstalk becomes a great factor for the demodulation error.
FIG. 6 shows a mechanism of generation of crosstalk in a wobble signal. Under normal conditions, when a laser beam spot is on the track of a track N on the optical disk, the laser beam spot is also radiated on adjacent tracks N−1 and N+1 as well as on the track N. A reproduced signal is superimposed on wobble signals of these tracks N−1 and N+1 also. However, a relationship among the phases of the wobble signals of the tracks N, N−1, and N+1 varies at all times. Moreover, modulation, such as MSK or HMW, is also added to respective wobble signal components. Hence, a wobble signal of the track N having undergone crosstalk from the wobble signals of the tracks N+1 and N−1 exhibits unpredictable phase variations.
Wobble signals complying with the Blu-ray standards employ both the MSK scheme and the HMW scheme, and a heterodyne scheme is adopted as a demodulation scheme. The scheme is one for detecting a modulated portion by means of multiplying a wobble signal by a clock signal that is obtained by supplying the wobble signal to the PLL circuit. Since the PLL circuit operates in accordance with a mean wobble frequency and a mean phase from a time constant of a loop filter, the clock signal generated by the PLL circuit cannot follow jitter induced by variations in crosstalk component.
FIGS. 7A to 7D show heterodyne operation performed according to presence/absence of crosstalk. FIG. 7A shows heterodyne multiplication of a crosstalk-free MSK-modulated portion. The MSK-modulated portion can be detected by integration of multiplied signal waveforms. FIG. 7B shows heterodyne multiplication of the MSK-modulated portion performed when a phase shift has arisen, for reasons of crosstalk, between the wobble signal and the multiplying clock signal. When compared with the operation shown in FIG. 7A, an envelope having a multiplied waveform is wavy, and difficulty is encountered to detect the MSK-modulated portion accurately. FIG. 7C shows heterodyne multiplication of the crosstalk-free HMW-modulated portion. FIG. 7D shows heterodyne multiplication of the HMW-modulated portion performed when a phase shift has arisen, for reasons of crosstalk, between the wobble signal and the multiplication clock signal. It is seen from the drawings that a disturbance arises in the envelope having a multiplied waveform because of a phase shift induced by crosstalk, thereby posing difficulty in detection of the HMW-modulated portion as in the case of the MSK-modulated portion.
With a view toward solving the problem, preparing a plurality of clock signals of different phases for the purpose of selecting a clock signal of the best phase is disclosed in the related art. For instance, JP 2005-216394 A describes a demodulator intended for effecting stable demodulation by means of enabling performance of more accurate phase control without involvement of an increase in sampling frequency. Specifically, there is described a configuration where a circuit that performs demodulation by subjecting an input modulation signal wave to computation along with an internal reference wave has, as a table for the internal reference wave, a plurality of tables whose phases are changed; where, even when a phase difference exists between a clock signal used for generating an internal reference wave and an input modulation signal, the phase of the internal reference wave is controlled so as to cancel the phase difference by means of a configuration that enables selection of any one from the plurality of tables; and where computation for demodulation can be performed by means of the in-phase waveforms.
However, such a configuration for preparing a plurality of clock signals having different phases becomes complicate, and cost increases, as well.